Apparatus for generating masses of hot ionised gas and amplifying random fluctuations



Sept. 27, 1966 R. FERRARI ETAL 3,275,868

APPARATUS FOR GENERATING MASSES 0F HOT IONISED GAS AND AMPLIFYING RANDOM FLUCTUATIONS Filed Nov. 1:5, 1963 From Gas pp y man,

fiTToRN Ys United States Patent C) 3,275,868 APPARATUS FOR GENERATING MASSES F HOT IONISED GAS AND AMPLIFYING RANDOM FLUCTUATIONS Ronald Leslie Ferrari, Stanmore, and Alan Reddlsh, Pm-

ner, England, assignors to The M-O Valve Company Limited, London, England Filed Nov. 13, 1963, Ser. No. 323,382 Claims priority, application Great Britain, Nov. 14, 1962, 43,087/ 62 1 Claim. (Cl. 313-63) This invention relates to method and apparatus for generating masses of hot ionised gas. Such methods may be of interest, for example, in connection with experiments on controlled thermonuclear reactions.

The invention consists in a method of generating a mass of hot ionised gas in which a stream of gas ions is caused to travel over a considerable distance in proximity to a medium which is capable of propagating waves of a type which can interact with Waves propagated along the stream of ions with a phase velocity directed in the same sense as, but of lower value than, the mean velocity of the stream. The system constituted by the stream of ions and said medium is such that by means of such in interaction there occurs amplification of waves which are set up in the system by virtue of thermal fluctuations of the motion of the ions. As a result, the thermal energy of the ions is increased at the expense of the energy of directed motion of the ions so that a mass of hot ionised .gas is obtained at the downstream end of the region over which the interaction occurs.

In effect, the invention is based on the concept of amplification of thermal noise present in the stream of ions by processes akin to those utilised in travelling wave electron tubes. The invention may be further explained as follows.

Consider a stream of gas ions moving with a mean velocity U. The stream may for example be generated by providing within a gas-filled vessel a first electrode system within which a gas discharge is arranged to occur and a second electrode system adapted to set up an electrostatic field which draws positive ions from the region of the discharge and accelerates them to the required velocity; the stream may be maintained in a coherent form by means of external fields, for example a uniform magnetic field directed parallel to the required direction of motion, or where the ions are positive, as will usually be the case, by the space charge neutralisation effects of an equal number of electrons, or possibly by a combination of both these means.

It is known that a variety of wave motions may be propagated along such a stream of ions. If, in a stationary ion system corresponding to the stream of ions, the phase velocity (at a given frequency F) for one particular type of wave motion has a value V less than U, then in the moving system there will be two possible corresponding waves respectively having phase velocities (relative to a stationary observer) of value (U+V) and (UV) both directed in the same sense as the motion of the stream; these two waves will be respectively referred to as the fast wave and the slow Wave.

Now suppose that the stream of ions travels over a considerable distance in proximity to a medium which is capable of propagating waves of a type which can interact with aslow wave propagated along the stream of ions, this medium being either stationary or moving in the same sense as the stream of ions with a velocity less than the phase velocity of the relevant slow wave, and being such that at the frequency F the phase velocity of the relevant type of wave in the medium is approximately equal to the phase velocity of the slow wave in the stream of ions.

ice.

Then any disturbance at the frequency F in the combined system constituted by the stream of ions and said medium will grow in space in the direction of motion of the stream due to interaction between a slow wave in the stream of ions and a substantially synchronous wave propagated in said medium in the same sense as the motion of the stream.

Examples of media which may support waves of suitable types for interaction with the stream of ions are as follows:

(a) A quasi-stationary plasma (that is a region of ionised gas containing substantially equal concentrations of positive ions and free electrons) disposed in a magnetic field directed parallel to the direction of motion of the stream of ions, this plasma possibly being generated by the action of the stream itself;

(b) A second stream of ions moving in the same sense as the first but at a lower mean velocity such that a fast wave in the second stream has the same phase velocity as a slow wave in the first stream;

(c) A delay line of a kind such as is utilised in travelling wave tubes.

For any specific system the interaction phenomenon will be observable over some finite range of frequencies, and in any particular case a knowledge of the types of wave involved and the coupling between them will make it possible to calculate the relevant range of frequencies and the gain obtainable Within this range by means of known thereory such as is applied to travelling wave tubes. In this connection attention is drawn to the following publications:

(1) Article by R. L. Ferrari beginning at page 1495 of the Proceedings of the Fifth International Conference on Ionization Phenomena in Gases, 1961, published by the North Holland Publishing Co., Amsterdam.

(2) Article by A. F. Haefi beginning at page 4 of vol- 1 me 37 (1949) of the Proceedings of the Institute of Radio Engineers.

(3) Article by J. R. Pierce beginning at page 980 of volume 37 (1949) of the Proceedings of the Institute of Radio Engineers.

(4) Pages 214 to 255 of the book by A. H. W. Beck entitled Space Charge Waves and published in 1958 by Pergammon Press.

It will be appreciated that the stream of ions will initially be at some finite temperature such that there are random thermal fluctuations of the motion of the ions in the stream. As is illustrated by Nyquists noise theorem (see Physical Review, volume 32, page such fluctuations may be regarded as constituting noise signals present in the stream of ions and any such signals having frequencies within the range for which the interaction process occurs will be amplified by the mechanism referred to above. As a result, the energy of random motion of the ions will be increased (at the expense of the energy of directed motion of the stream), so that a mass of hot ionised gas will be obtained at the downstream end of the region over which the interaction occurs. It will be further appreciated that, within limits imposed by the occurrence of phenomena other than the interaction process referred to above, the longer the interaction region the hotter will be the resultant mass of ionised gas.

One method in accordance with the invention will now be described, by Way of example, with reference to the accompanying drawing which shows an apparatus for carrying out the method.

Referring to the drawing, the apparatus includes a hollow, generally cylindrical, glass envelope 1 within one end of which is housed an ion gun 2 arranged to direct a beam of positive ions along the envelope, towards a and a pair of disc-shaped metal cathodes 7, of diameter 5 centimetres, disposed closely adjacent the anode 6, one at each end of the anode 6, in planes perpendicular to the axis of the anode 6. All but one of the cathodes 7 have circular apertures 8 of diameter. one centimetre at their centres. The ion sources 4 are disposed :within the envelope 1 at a Spacing of one centimetre from one another with their anodes 6 in line and coaxial with the envelope 1,-the cathode 7 not having an aperture at its centre being furthest from the collector electrode 3.

The accelerator electrode 5 is dispose-d betweenthe ion sources 4 and the collector electrode 3 and comprises ametal tube 9, having an outwardly extending radial flange at one end, the electrode 5 being disposed coaxially within the envelope 1 with the flange 10 at a spacing of one centimetre from the nearest cathode 7. The tube 9 has a length of 4 centimetres and an internal diameter of 1 centimetre and the flange 10 is of such a diameter as .to extend to the internal surface of the envelope 1.

The collector electrode 3 comprises a metal disc of diameter 5 centimetres disposed in a plane perpendicular to the axis of the envelope 1 at a distance of 25 centimetres from the accelerator electrode 5.

Leads 11 to the various, electrodes 3, 5,? 6 and 7 are sealed through the envelope 1, these leads 11 also serving, where necessary, to support the electrodes.

In use of the apparatus, hydrogen gas is admitted continuously to the interior of the envelope 1 via an inlet 12 adjacent the ion gun 2 and the interior of the envelope 1 is continuously evacuated via an outlet 13 positioned half-way along the envelope 1. By this means the'pres sure within the part of the envelope 1 on the collector electrode side of the flange 10 is maintained at a value below 10 'torr and a relatively high pressure of the order of 1-10 millitorr is maintained within the remainder of the envelope 1.

In addition, the apparatus is disposed within a magnetic field directed axially along the envelope 1, as indicated by the arrow H in the drawing, and the various electrodes 3, 5, 6 and 7 are connected to four suitable constant 'voltage' sources 14, 15, 16 and 17 so as to causeto be projected along the envelope 1 from the ion gun 2 to the collector electrode '3 a beam of hydrogen ions in which there are elfectively two streams, one at a current of 1 milliampere and accelerated to 1.5 kilovolts, and the other at a current of 8 milliamperes and accelerated to 6 kilovolts. To this end, two of the voltage sources.

14 and 15 are connected so as to maintain the cathodes 7 of each ion source 4 at the same negative potential with respect-to the. anode 6 of that ion source 4, this potential having a value of theorder of one. kilovolt such that a suitable. discharge occurs in each ion source 4.- Positive ions are drawn out of these discharges to form the required beam by arranging that a third voltage source 16v is connected so as t-omaintain the .cathodes 7 of the ion source 4 nearer the accelerator electrode 5 at a negative potential with respect to the cathodes 6 7 of the other ion source 4, this potential nominally having a value of about 4.5 kilovolts, and a fourth voltage source 17 is connected so as to maintain the collector example,

. thermal energyof the ions .at the t 4i electrode 3 and the accelerator electrode 5 at the same negative potential with respect to the cathodes 7 of-the ion source 5 nearer the accelerator electrode 5,this poten-, tial nominally having a value of about 1.5 kilovolts.

of the ions drawn from the ion source 4, 'which fluctua-v tions give rise to various waves propagated along both the component streams of the beam, the frequencies of these waves extending over a wide range. It may be shown that in the apparatus described above,

in both component streams of the beam increases as theytravel from the accelerator electrode 5 to the collector electrode 3, producing a mass of hot-ionised .gas at the downstream end of the envelope 1.

From theoretical calculations, a. maximum gain of the order of 30 decibels may be expected at a frequency" of 8.9 megacycles per second, the gain decreasing as the frequency departs in either direction 'from this value.

We claim: An apparatus for generating a mass of hot ionised gas comprising: an enclosure containing a gas at a llOW pres sure; a collector electrode and an ion gun housed in said enclosure, the ion gun including two ion sources. each consisting of an electrode system generating a gas dis-'- waves having phasevelocities greater and smaller than:

the mean velocity of that stream, and the magnitudes of the biasing potentials being such that the streams travel at mean velocities dilfering by an amount such that said waves of smaller. phase velocity setup in the faster stream due to said thermal fluctuations interact with said waves. of greater. phase velocity set up in the slower stream due to said thermal fluctuations, whereby, due to such interaction, there occurs travelling Wave tube type amplification of the waves set up in the systern due to said thermal fluctuations, thereby increasing. the

of-directed motion of the ions.

References Cited by the Examiner UNITED STATES PATENTS 2,801,362 7/1957 Hevenstreit 315-5.14 3,138,019 6/1964 Fonda-Bonardi 31363 X.

FOREIGN PATENTS 646,015 7/1962 Canadap JAMES W. LAWRENCE, Primary Examiner. GEORGE N. WESTBY, Examiner. S. SCHLOSSER, R. SEGAL, Assistant Examiners.

-by way of any slow waveof frequency less than 14.5 megacycles propagated along the fasterion stream interacts with a fast wave propagated along the slower stream. in such a manner as to amplify the interacting waves.v As a result, the energy of'random motion of the ions expense of the energy 

